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Armin usually wears either the first year school uniform, or a white button up shirt and dark pants. He also goes everywhere with something covering his head; initially it was his futon from home, although Armin has recently been seen also with either a sheet, a blanket or sweater tied around his head.
Very shy and timid, Armin usually spent his time studying role-playing games and learning about fantasy worlds. He also has a very active imagination and imagines the futon he wears (which he named Bigbee) as a magical cloak. Armin has claimed to Eren, Mikasa and Levi that it is part of him and if he does not have it, he becomes ill. He is also one of the brightest students in his class, willing to help others understand difficult subjects like math; several have noted that he is also very nice and modest to those around him.
After letting down his team in a dodge ball tournament during their last year of elementary school, Armin remained home the entire summer; he told his grandfather to not let anyone see him and stayed locked up in his room. Armin engrossed himself into fantasy games and frequently developed new worlds he would imagine himself exploring. Also at this time, he would envision his futon as a security blanket (or magic cloak) and begin wearing it constantly.
After several days, the entire class has noticed Armin has yet to show up for school. Hearing that a mandatory dodge ball tournament will be held the following day, both Eren and Mikasa arrive to his house to convince him to come. Armin steadfastly refuses to and does not believe Eren's claims of honesty, believing it to be part of a prank. Mikasa convinces him to let them inside and they find Armin wrapped up in a futon. He again refuses to and recalls the events from the previous dodge ball game he was part of. After some prompting from Mikasa, however, he then changes his mind and agrees to go to school the next day and take part in the tournament.
The following day, Armin does come to school, surprising the teacher and all the other students. Still wrapped in his futon, Armin takes part in the tournament with the others; he notes that since Eren has not shown up, Mikasa's performance has dropped roughly 30%. He mentions this to Jean, who is bewildered by it. Despite this and several other setbacks, Class 4 advances to the next round. Armin yells out for Eren sleeping in and soon after, Armin's futon is stolen by Mikasa, who wraps herself in it. Annie, one of their next round opponents, comes over and asks where Eren is; Armin claims Eren has been in the restroom all day. Mikasa's negative aura is beginning to be felt by the others and Armin proposes disguising Jean as Eren due to them looking similar; this fails when Mikasa notes she cannot sense Eren's ki and remains depressed. Fortunately, Eren eventually makes it back in time and Armin quickly reclaims his futon from Mikasa. As the match gets underway, Armin is eliminated by Annie but their side is able to win the match.
Along with the rest of his classmates, Armin decides to join the Survey Club and is assigned to be in the 'Wall Beautification Club' as well. During a trip to Wall Rose, Armin witnesses the challenge between the Survey Club and the Student Council; when Jean disputes his loss to Hitch, Armin notes that it still counts as a win despite the unconventional means. As they get their results back from a test, Armin hears Sasha worry about her grades and offers to help tutor her after school. Despite some initial miscommunication, Armin is able to reach Sasha and some progress is shown on her end due to his efforts. He hears about an upcoming dance and, wanting to go with someone he trusted, begged Eren to pair up with him. Only after he insists that he is not good at this kind of situation does Eren finally relent. He later defuses a brewing situation between Mikasa and Annie, not realizing they both had plans to invite Eren to the same dance event. Armin later helps take part in the contest between the first year students and the upperclassmen. He tries to come up with plans to help Eren lead them to a victory; ultimately though, the first years lose and are forced to attend the dance with the Titan students.
During the summer months, Armin returns to the school with the others to help Levi in cleaning. Levi notices Armin still wearing his blanket and orders him to remove it. Armin does not want to comply and tries to explain that the heat does not affect him. This does not work and Armin is prompted by Eren to reveal that he does not feel like himself without it. After hearing Levi continue to insist he remove it, Armin sees no other solution and begrudgingly agrees. As he starts to take it off, ultimately Armin finds he cannot do it and Eren urges him to run away. Armin does so, causing Levi to swear he will remove it by any means necessary.
Levi manages to trick Armin into leaving his futon unattended and steals it from his balcony. Armin immediately calls Eren, who arrives with Mikasa, Jean, Sasha and Connie. Already beginning to show the symptoms of a cold from not having his futon, Armin informs them that his room was much cleaner than it was before his futon went missing and believes Levi to be responsible for the theft; he begs them to find Levi and steal his futon back. He goes with them to the student dormitories where Levi is staying and is caught by surprise by the traps seemingly laid out by him. Initially viewing it as hopeless, Armin suddenly has a change in heart and rallies the others forward. One by one, they all fall victim to traps until only Armin is left to confront Levi; bursting through the door, Armin sees Levi holding both his futon and a long needle; this causes Armin to scream and lunge towards Levi to grab his futon back. After Eren and Mikasa both arrive, they find Armin happily curled up underneath his futon. After Levi explains that there was a hole in it and he merely took Armin's futon to sew it up, Armin apologizes and says that Levi is a great person after all.
- Eren Yeager - Armin and Eren have been friends for their entire lives. Despite his claims of innocence, Armin initially distrusted Eren's claims during the first week of school but would later see that he was just trying to help. Armin asks Eren to be his partner at a dance, knowing that he was one of the few people Armin genuinely trusted.
- Mikasa Ackerman - While they can occasionally go against each other, Armin views Mikasa as another of his best friends. He can recognize when she is feeling depressed and immediately tries to find a way to relieve her of the fatigue.
- Sasha Blouse - Attending the same class together, Armin has a steady relationship with Sasha. He volunteered to help Sasha with her issues with math, showing that he cares about her performance in school.
- Levi - Initially intimidated by Levi's stoic attitude, Armin soon earns Levi's ire for wearing his futon during the summer. Armin develops a brief hatred for Levi after he steals Armin's futon but once it is discovered Levi only stole it to repair a hole in it, Armin's thoughts of him change and views Levi in a much more favorable light.